IMPEDANCE SPECTROSCOPY OF SELF-ASSEMBLED MONOLAYERS ON AU(111) - EVIDENCE FOR COMPLEX DOUBLE-LAYER STRUCTURE IN AQUEOUS NACLO4 AT THE POTENTIAL OF ZERO CHARGE

The impedance of a Au(111) electrode modified by three different self- assembled monolayers has been studied over a wide frequency range in a queous NaClO4 solutions. The impedance characteristics of the bare ele ctrode are compared with those of the same electrode modified with dec anethiol, omega-hydroxydecanethiol, and 4'-hydroxy-4-mercaptobiphenyl . In the case of the decanethiol system the interfacial impedance can be represented as a capacitor due to the self-assembled monolayer in s eries with the solution resistance. However, for the latter two system s, the impedance behavior is more complex with a high resistance in th e monolayer parallel to the expected capacitance. This behavior is qui te different than that discussed to date in the literature and assumed in the interpretation of voltammetric data for modified electrodes, T he parameters for these systems are reported here as a function of ele ctrolyte concentration at the point of zero charge on the bare Au(111) . The consequences of the present observations are discussed with resp ect to the phenomena observed for these systems.